1. Field of the Invention
The present invention relates to an ultraviolet-curable ink composition for ink jet recording and, more particularly, to an ultraviolet-curable ink composition for ink jet recording containing titanium oxide as a colorant, the composition having excellent ejection stability and providing excellent visibility of printed matter having a base material which forms a black or dark background.
2. Description of Related Art
In a printing method using an ink jet recording apparatus, ink is ejected from nozzles and deposited onto a recording material, and satisfactory printing can be performed on a curved surface or a surface having irregularities because the nozzles do not contact the recording material. Therefore, wide ranges of industrial applications are anticipated for this printing method.
Inks used in an ink jet recording system wherein a dye is used as a colorant includes, for example, water-based inks wherein water is used as a prime solvent, and oil-based inks wherein an organic solvent is used as a prime solvent. Among these inks, water-based inks prepared by dissolving a dye in an aqueous solvent are mainly used. However, when using a conventional water-based dye ink for industrial applications, there are problems, for example, slow drying rate of ink on a non-absorbing material, poor adhesion of printed images, and insufficient durability such as abrasion resistance, water resistance and light resistance. Inks proposed to solve these problems include, for example, water-based and oil-based inks for ultraviolet-curable ink jet recording wherein various color pigments are used as a colorant, said inks being cured and dried by irradiation with radiation such as ultraviolet light.
Among these inks, it has been proposed that inks used for white printing on a base material which forms a black or dark background consist of, for example, an ultraviolet-curable ink composition for ink jet recording wherein titanium oxide, which is a white pigment having excellent covering properties, coloring power, and chemical resistance, is used as a colorant. For example, Japanese Unexamined Patent Application, First Publication No. Sho 62-64874 proposes an ink for an ink jet printer of a continuous system wherein a thermoplastic resin is used as a dispersion medium of a pigment and rutile titanium oxide is used as a white pigment; Japanese Unexamined Patent Application, First Publication No. 2000-336295 proposes an ink wherein the dispersion stability and ejection stability are improved by using anatase titanium oxide; and Japanese Unexamined Patent Application, First Publication No. 2001-207098 proposes an ink wherein rutile titanium oxide and a pigment dispersant are used.
As described above, the titanium oxide is suited for use as a pigment for white ink because of its excellent covering properties and coloring power. However, titanium oxide is liable to cause precipitation of the pigment when an ink jet recording apparatus is stopped for a long period because of its large specific gravity. If the pigment has poor redispersibility, it precipitates in the ink supply path, and thus the ink becomes non-uniform, which causes changes in its physical properties and deteriorates the ejecting performance. Furthermore, the pigment precipitated in an ink supply tank agglomerates to form a hard cake and the original dispersion state cannot be recovered even if a redispersion operation is performed by stirring. Therefore, in order to obtain an ultraviolet curable white ink for ink jet recording which has excellent ejection stability and which provides excellent visibility of printed matter having a base material which forms a black or dark background, it is important to stably disperse the titanium oxide in a mixture containing a photopolymerizable compound without causing precipitation of the titanium oxide. If the titanium oxide is precipitated, it must be excellent in redispersibility of the precipitated titanium oxide. However, a conventional ultraviolet-curable ink composition for ink jet recording, wherein titanium oxide is used as a colorant, is not excellent in the characteristics involved in dispersion or redispersion of the titanium oxide, and is also insufficient in ejection stability.